The present invention relates to an improved process for removing turbidity-causing impurities from surface waters and other domestic and industrial water supplies containing such impurities.
Turbid water has a haziness caused by insoluble, suspended particles such as clay, silt, bacteria, viruses, organic debris resulting from the decay of plant life and similar materials present in the water. Turbidity standards have been established by governmental regulatory authorities for determining the water quality of municipal distribution facilities. With certain limited exceptions, a monthly average of one turbidity unit (TU) has been set as the maximum contaminant limit for turbidity. Turbidity measurements are made by determining the amount of light that is scattered by particulate matter in a sample of water.
Although water treatment for turbidity removal may vary depending on raw water quality, conventional systems generally involve pre-treatment including coagulation/flocculation and sedimentation, followed by filtration and chlorination.
Coagulation is ordinarily carried out in a rapid mixing tank by adding to the raw water a chemical agent which causes agglomeration of the suspended matter into larger particles that can settle to the bottom of a containment means. The agglomerated particles, or floc, are sometimes subjected to gentle agitation in order to form floc bundles large enough to settle rapidly. A separate flocculation tank is often provided for this purpose.
Sedimentation is the process by which floc is separated from water by precipitation and deposition, and depends on the effect of gravity on particles suspended in a liquid of lesser density. Sedimentation without pre-treatment is rarely adequate for clarification of turbid water, as it does very little for removal of such fine particulate substances as clay, bacteria and the like. Also, sedimentation results in sludge formation and thus requires means for disposal of the sludge.
After the impure water undergoes appropriate pre-treatment, it is filtered for the removal of suspended particles by passage through a porous medium. Most conventional water treatment systems employ granular media filtration. For example, rapid sand filters and mixed media filters (e.g. sand and particulate coal) have proved effective for reducing turbidity. However, granular media filtration virtually always requires chemical pre-treatment for effective turbidity removal. Slow sand filters, which were among the first water treatment systems to be used for large scale filtration, have numerous shortcomings when it comes to treating turbid water, including low filtration capacity, channeling, and ease of clogging.
The cost of the space and equipment required for installation of a conventional water treatment system of the type described above is considerable, as is the continuing cost of operation and maintenance of such a system. Although efforts toward improving these prior art processes have continued, the improvements have related in general to enhancing floc formation and sedimentation, reducing the load on the filters, and using larger grained sand and higher filtration rates as a means of reducing cost.
Alternative municipal water treatment processes employing filter-aid principles have been developed more recently to reduce the requirements for large and expensive vessels to provide for floc formation and sedimentation. These systems are commonly known as precoat filtration systems and consist essentially of a pressure vessel containing a porous septum which is coated on the upstream side with a powdered filter medium which forms the precoat filter cake. As water flows through the cake, the solid impurities present in the water are caught in the cake's small pores. In this system, additional filter aid is normally added to the body of the raw water in an amount sufficient for efficient and effective turbidity removal, the added filter aid being referred to as a body feed. The amount of body feed must be such as to prevent filter binding and waste of precoat material resulting from short periods of operation. In general, optimum results are obtained using about 0.8% to 1.0% of body feed by weight, in accordance with the manufacturers' recommendations. The most widely used of the precoat filters is the diatomaceous earth filter.
It has been reported that effective clarification and purification of turbid water may be achieved by precoat filtration, using relatively small amounts of diatomaceous earth as body feed, provided that chemical additives, namely floculating agents and polyelectrolytes, are added to the raw water with the body feed. See, U.S. Pat. No. 3,227,650. Apparantly, the use of polyelectrolytes in this patented process is essential, for if a flocculating agent alone is used in conjunction with the diatomaceous earth, the water is practically unfilterable. Further, it has been found that if diatomaceous earth alone is used as the precoat and body feed, the filtrate, while reduced in turbidity, is unacceptable for potable or industrial use.
While recent developments in the art have eliminated some of the cost of installation and equipment maintenance, the overall operating cost of municipal water treatment remains relatively high. In short, the cost of water treatment, whether by conventional systems or by systems operating on the filter aid principle, makes it uneconomic for many smaller sized municipalities and industries which might advantageously employ a water treatment plant. Hence, the development of an efficient and effective process for producing substantially pure, clear water for domestic and industrial consumption at a reasonable cost continues to be a highly desired objective.